1. Field of the Invention
This invention relates generally to communication systems using spread spectrum Pseudo-Noise (PN) coding techniques, and pertains more specifically to methods and systems for PN encoded signal prioritization and collision mitigation.
2. Prior Art
A variety of multiple access communication systems has been developed for transferring information among a large number of system users. Techniques employed by such multiple access communication systems include time division multiple access (TDMA), frequency division multiple access (FDMA), and AM modulation schemes, such as amplitude companded single sideband (ACSSB), the basics of which are well known in the art.
In Spread Spectrum (SS) or TDMA-SS transmission systems, a succession of short-duration bursts emanating from a number of different stations are presented to a demodulator. Each burst may contain data frames from one or more data channels. Each data frame generally contains a synchronization or sync word and a data payload area.
The TDMA structure is composed of a stream of frames with a number of fixed-time slots per frame. Each time slot may be of an assigned type: entry and registration, routine maintenance, priority messages, mass data transfer, and interrupt. The composition of slot types in a frame may be reassigned from frame to frame. A time slot in a frame may be assigned to one specific user; or a time slot may be a free-for-all slot; any number of users may attempt to use it on a first-come, first-serve basis.
Frequently, a class of users may need to communicate messages on an ad-hoc basis during a time slot that is reserved for ad-hoc messages, e.g., an interrupt time slot. The interrupt time slot is not assigned a priori to any specific user, but is available to all users on a free-for-all basis. For the case when multiple users occasionally transmit a message during the same interrupt time slot, the possibility exists that the different user transmissions will arrive nearly simultaneously at the receiver, thus “colliding” and interfering with each other. Prior art approaches design the spread spectrum correlation receiver to demodulate the received signal that arrives first in time and to reject other signals that are outside the correlation window of the correlation receiver. One disadvantage of this method is that higher priority messages from one user may be rejected in favor of lesser priority message from another user. Another disadvantage is that all messages received in the interrupt time slot may be rejected if multiple received spread spectrum signals arrive within the receiver's correlation window (e.g., within 2 PN code chips) of each other.
Therefore, a signaling method is desired that will enable interrupt message priority to be assigned to different users (i.e., Spokes) of the system and for the receiver (i.e., Hub) to automatically “sort” the messages in the correlation receiver to automatically select the highest priority message. Messages having lesser priority are thus automatically rejected in favor of a higher priority message.